ScrumBut is a practice where teams implement partial apsects of Scrum. As in - "We did Scrum BUT we did not have daily standups". Scrum pracitioners look down on teams that practice ScrumBut and perhaps for good reason. As they argue, when a team does not implement a prescribed practice in Scrum, the team is allowing for a problem to fester.
For example, if a team finds that a Product Manager is unable/unwilling to participate in an agile team as a "Product Owner", using ScrumBut, the Business System Analyst may take on that role. The "Product Owner" is one of three key roles that Scrum prescribes. Not having the real Product Owner play that role is a violation of Scrum. In reality, there might be organizational hurdles, as in highly matrixed organizations, that prevent the Product Manager from participating in agile teams. Should the agile team give up their move towards adopting agile methodologies?
Perhaps not. ScrumBut is an excellent way to get started with agile methodologies. Agreed that it is not likely to give the same amount of return that Scrum promises. However, it is an incremental way to adopting Scrum. It is likely to result in less of a culture shock to new teams and may provide the breathing space to winning hearts and minds.
The end goal though must be to eliminate the "But" and have just "Scrum". Assuming of course, Scrum is what you want for an agile methodology.